This invention relates to the manufacture of electrical circuits formed on a surface of a substance of insulating plastics material
Modern, so-called "engineering plastics", are increasingly being used as parts, sub-assemblies or structural members in all kinds of manufactured goods; such as motor cars, aircraft, "white" goods, telecommunications equipment There is often a requirement to incorporate an electrical circuit on such parts and, hitherto, this has been achieved by printing, bonding or otherwise applying the circuit onto the surface of such parts to effectively produce a "printed" circuit on an external surface of a three-dimensional object The printed circuit may include integrally formed resistors, capacitors and inductors and may also have other devices added to the circuit.
Electrical circuits of this kind can only be applied to an external surface of an object, are prone to physical damage and can exhibit the undesirable characteristics of inter-conductor contamination by handling, solder flux residues and accumulated debris, such as dust, in use, which will all reduce the insulation resistance and dielectric properties or give rise to variation in these properties.
U.S. Pat. No. 4,415,607 (Denes et al); assigned to the Assignee of this application, discloses a method of making a printed circuit network device including the steps of depositing an insulating primary substrate layer (a screen printed layer of titanium dioxide loaded epoxy resin dissolved in a combined butyl Cellosolve acetate and n-butanol solvent) on a temporary support structure having a release surface (a super-calendered, clay-coated one side natural kraft paper, coated on the clay side with a silicone release coating), applying to the exposed surface of the primary substrate layer certain defined conductor areas, (screen printed conductive ink using the same epoxy resin based vehicle loaded with a conductive material), mounting the temporary support member with its several layers (together forming a decalcomania) in facing relationship with a permanent support member, and thereafter releasing and removing the temporary support member from the mounted primary support layer.
The sole given application is for the production of printed circuit network devices incorporating resistive paths and, for this, the method additionally comprises the steps of applying defined resistor areas on the exposed surface of the primary substrate layer and in electrical connection with the conductor areas and trimming the respective resistor areas to a predetermined resistive value. An insulating permanent base or support being injection moulded directly to the decalcomania.